Fibers and pulps for papermaking based on chemical combination of poly(acrylate-co-itaconate), polyol and cellulosic fiber

ABSTRACT

Disclosed is a fiber comprising, chemically bonded together, (a) a conventional cellulosic fiber, such as a Kraft fiber or a chemithermomechanical pulp fiber; (b) poly(acrylate-co-itaconate), such as the acid form of a poly(acrylate-co-itaconate) comprising 90-95 mole % acrylate and 5-10 mole % itaconate having weight average molecular weight of about 600,000-900,000; and (c) a polyol, such as polyethylene glycol; also disclosed are methods for making such fibers, especially evaporatively depositing an intimate mixture of the copolymer and polyol on the fiber followed by thermally crosslinking at specific temperatures for limited periods; absorbent paper which can be made by wet-laying the fiber, especially in admixture with conventional fiber; and derivative paper structures, such as multi-ply disposable absorbent towels.

FIELD OF THE INVENTION

This invention relates to fibers, such as Kraft fibers, which arechemically modified with certain poly(arcylate-co-itaconate) copolymersand polyols; to chemical methods for making such fibers; to improvedpaper which can be made by wet-laying the fibers, especially as a pulpin admixture with conventional papermaking pulps; and to derivativepaper structures, such as multi-ply disposable absorbent towels.

BACKGROUND OF THE INVENTION

The is an ongoing interest in the development of absorbent articles suchas paper towels. Disposable paper towels are widely used in the home forwiping spills, especially of water or watery liquids; for cleaningwork-surfaces such as those of the kitchen and bathroom; for foodpreparation and handling; or for cleaning glass. More generally,absorbent papers are sometimes incorporated into other absorbentarticles, such as dressings, catamenials and disposable diapers.

Manufacturing, more specifically sheet-forming, processes for paper arewell-established in commerce. Papermaking machinery is verycapital-intensive, and as a result, improvements in absorbent paperwhich do not require any major change in, or complication of, thepaper-forming process tend to be highly appreciated. The processes ofmajor importance include air-laying and wet-laying.

In outline, the latter process involves filtering a dilute dispersion offibers onto a mesh (usually termed a Fourdrinier wire) and drying theresulting web. There is a large installed base of manufacturingequipment using continuous, high-speed machinery based on the wet-layingtechnique, representing considerable investment.

Conventional papermaking fibers suitable for wet-laying papermaking arecellulosic fibers which disperse well and can readily be filtered anddried. They typically absorb relatively small amounts of water, of theorder of a few grams per gram of bone dry fiber.

The simplest notion for improving the absorbency of paper involvesadding thereto a highly water-absorbent material, such as one of thegel-forming polycarboxylate polymers, which are well-known in the art.Very high absorbencies are possible, of the order of hundreds of gramsof water per gram of polymer. Such materials have found particularutility as disposable diaper "superabsorbents".

Superabsorbents are however inherently very difficult to handle in awet-laying process. Thus they tend to disintegrate under the relativelyhigh shear forces involved in wet-laying papermaking. Moreover, they aredifficult to filter, tending to block the Fourdrinier wire; and oncedeposited, they are very difficult to dry. The final product tends to bestiff and may not rewet to an acceptable degree when in use.

Applying the above-identified simple absorbency-improving notion toprocesses other than wet-laying has led to the development of alaminated structure or "sandwich" having outer plies of conventionalpaper and an inner layer consisting essentially of superabsorbent.However, the superabsorbent tends to leak out from the product paperstructure, especially through pinholes or when the paper structure istorn. Slippery, gel-like material is released, which is a seriousaesthetic disadvantage.

The deficiencies of the above approaches suggest a need to consider morethan just the absolute magnitude of the absorbency which can be had froma particular paper additive. Thus, although absorbency is of primaryimportance, other requirements, such as ease of manufacture and productaesthetics, must be met. In addition, as distinct from water-absorbingcapacity, another problem which has been identified in the context ofabsorbent structures has to do with rate of water uptake, in technicalterms, "wicking rate". Wicking rate is particularly important in adisposable paper towel which must quickly absorb a spill.

In principle, it is possible to suggest making absorbent paper bywet-laying an improved absorbent fiber (as distinct from particulatesuperabsorbents on one hand or conventional papermaking fibers on theother). However it would seem that these reports of "absorbent fibers".However it would seem that these often involve mere physical coating ofa fiber, such as processes involving precipitating absorbent polymersonto fibers or polymerizing monomers such as acrylic acid andmethylenebisacrylamide in the presence of a fiber. In such situations,the chemical means are not present to covalently attach the polymer tothe fiber. In consequence, the coating may not survive the shear forcesinvolved in typical wet-laying operations, and may come off, the resultbeing wire-blocking and/or drying problems similar to those mentionedabove.

Grafted fibers are also well-known. Typical of grafted fibers are thosemade by graft copolymerizing methyl acrylate and cellulosic fibers inthe presence of an appropriate catalyst such as cerium (IV) ammoniumnitrate followed by hydrolyzing to the absorbent form. Absorbent graftedfibers are often not as strong as might be desired for wet-layingpapermaking, since the low molecular weight monomer used in thepreparation is capable of penetrating the fiber, polymerizing in theinterior, so that when hydrolyzed and exposed to water, the fiber"balloons" internally and can easily shatter.

Various highly absorbent polymers have been extruded, and the extrudateshave sometimes been termed "fibers". However, these materials are infact not fibers in the usual sense of cellulosic papermaking fibers,rather they tend to be chemically homogeneous, and as with the commonparticulate superabsorbents, form slippery gels and encounter processingproblems when wet-laid.

Oddly, to add to the above, there are reports in the literature ofvarious chemicals apparently similar to those used herein apparentlyimparting wet-strength and/or hydrophobicity, i.e., water-resistance, topaper.

In view of the foregoing considerations, improvements in absorbentcellulosic fibers which do not make the fiber incompatible withwet-laying are highly desirable.

Accordingly, it is an object of the instant invention to provide awet-layable papermaking fiber and pulp having an improved absorbentform.

More specifically, it is an object herein to provide a chemicallymodified fiber (hereinafter "the fiber of the invention") having threechemically bonded components, namely a cellulose of natural origin (suchas an ordinary pulp fiber), a poly(acrylate-co-itaconate) and a polyol;which fiber has a water-absorbent chemical form (such as the sodium saltform), which is useful especially in that it is readily capable of beingdistributed into a web by wet-laying (e.g., as a pulp) in admixture withuntreated fibers.

It is another object of the invention to provide absorbent wet-laidpaper comprising the fiber of the invention.

A further object of the invention is the provision of a suitableprocess, unreliant on metal catalysts as used in common graftingprocesses of the art, for reproducibly making the fiber of theinvention.

These and other objects are secured, as will be seen from the followingdisclosure.

BACKGROUND ART

For general discussion of coatings and chemical modifications ofpapermaking fibers and of paper see "Pulp and Paper, Chemistry andChemical Technology", Ed. James P. Casey, Wiley-interscience, 1981,Vols. I-IV. See also "Chemical Modification of Papermaking Fibers", K.Ward, Marcel Dekker, N.Y., 1973.

Japanese Laid-Open 50-5610, Jan. 21, 1975, discloses treating apreformed paper web with an aqueous solution containing polyvinylalcoholand various copolymers, especially maleic anhydride-methyl vinyl ether,followed by drying and thermally treating, to form high-wet-strengthpapers.

Papermaking wet-strength resins based on half-amides derived from maleicanhydride copolymers with various monomers are disclosed in U.S. Pat.No. 4,391,878, Drach, issued Jul. 5, 1983.

Papermaking sizing agents and adhesives based on carboxylated vinylpolymers are disclosed in U.S. Pat. No. 3,759,861, Shimokawa, issuedSep. 18, 1973.

Gantrez AN Technical Data Sheet, GAF Corp., suggests a number of usefulapplications for Gantrez polymers in connection with papermaking.Notably, Gantrez is suggested for use "as a beater additive to improvesizing, strength and dimensional stability." Further, "as a papercoating component, it can improve moisture . . . resistance."

U.S. Pat. No. 4,018,951, Gross, issued Apr. 19, 1977, disclosesabsorbent films prepared by heating methyl vinyl ether-maleatecopolymers with crosslinking agents such as diglycidyl ethers ordihaloalkanes. The films can assertedly be used in absorbent articles.

U.S. Pat. No. 4,128,692, Reid, issued Dec. 5, 1977, disclosesprecipitating absorbent polymers onto fibers from an aqueous slurry.

U.S. Pat. No. 4,721,647, Nakanishi et al, issued Jan. 26, 1988,discloses an absorbent article comprising hydrophobic fibers and awater-absorbent polymer as spherical particles.

U.S. Pat. No. 4,295,987, Parks, issued Oct. 20, 1981, discloses atwo-ply paper towel containing powdered absorbent copolymers. A layercan be sandwiched between two paper plies.

Brandt et al, U.S. Pat. No. 4,654,039, issued Mar. 31, 1987, reissued asRE 32,649 on Apr. 19, 1988, disclose superabsorbent polymers which canbe used in absorbent structures.

Weisman, U.S. Pat. No. 4,610,678, issued Sep. 9, 1986, disclosesair-laid absorbent structures comprising a mixture of hydrophilic fibersand discrete particles of a water-insoluble hydrogel.

Saotome, EP-A 192,216, published Aug. 27, 1986, discloses awater-absorbent fibrous structure, characterized as comprising a fibrouscellulosic material impregnated with a water-absorbent acrylic polymerand a fibrous material, which is produced by a method in which anaqueous solution of a monomeric component comprising acrylic acid and aradical initiator is diffused in a fibrous cellulosic material andheated, followed by blending with a fibrous material.

See also U.S. Pat. No. 4,354,901, Kopolow, issued Oct. 19, 1982 and U.S.Pat. No. 4,552,618, Kopolow, issued Nov. 12, 1985. The Kopolowdisclosures relate to compression or heat treatment of boards in the drystate after a wet-laying papermaking process. The boards comprise"hydrocolloidal fibers" such as those of U.S. Pat. No. 3,889,678,Chatterjee et al, issued Jun. 17, 1975.

Heat treatment of absorbent carboxyalkyl cellulose fibers in anabsorbent structure to derive improved fluid absorptive properties isdisclosed in U.S. Pat. No. 3,858,585, Chatterjee, issued Jan. 7, 1975.

Grafted, hydrolyzed absorbents are disclosed in "The Chemistry andTechnology of Cellulosic Copolymers", Hebeish, Springer-Verlag, 1981;see also U.S. Pat. No. 3,366,582, Adams et al, issued Jan. 30, 1968 andU.S. Pat. No. 4,151,130, Adams issued Apr. 24, 1979.

U.S. Pat. No. 4,151,761, Schoggen et al, issued Feb. 24, 1981, disclosessheets prepared from certain modified fibrous carboxymethylcellulosederivatives, sometimes known as bibulous cellulosic fibers. Such sheetsare disclosed in patents including U.S. Pat. No. 3,678,031, Schoggen,issued Jul. 18, 1972 and U.S. Pat. No. 3,589,364, Dean and Ferguson,issued Jun. 29, 1971.

U.S. Pat. No. 4,780,500, Sinka et al, issued Oct. 25, 1988, discloses acoating composition for paper and paperboard containing pigment, binder,lubricant and water. The composition comprises a copolymer of 80%-98%(wt.) acrylic acid and 2%-20% (wt.) itaconic acid. The copolymer iswater dispersible, has a molecular weight of 100,000-800,000 and is inacid, sodium, potassium and/or ammonium salt form. Included arecompositions comprising by way of copolymer 95% (wt.) sodium acrylateand 5% (wt.) diammonium itaconate and having M_(W) 250,000-400,000. Suchcopolymers can be used at a low level (0.05%-0.8% wt.) based on solidsin the coating composition as a retention aid to retard release of waterfrom the coating composition without increasing its viscosity.

Patents relating to papermaking processes generally useful in thecontext of the present invention and incorporated herein by referenceinclude U.S. Pat. No. 3,301,746, Sanford et al, issued Jan. 31, 1967;U.S. Pat. No. 3,905,863, Ayers, issued Sep. 16, 1975; Morgan, Jr. et al,issued Nov. 30, 1976; U.S. Pat. No. 4,191,609, Trokhan, issued Mar. 4,1980; U.S. Pat. No. 4,300,981, Carstens, issued Nov. 17, 1981; U.S. Pat.No. 4,440,597, Wells et al, issued Apr. 3, 1984; U.S. Pat. No.4,469,735, Trokhan, issued Sep. 4, 1984; and U.S. Pat. No. 4,637,859,Trokhan, issued Jan. 20, 1987.

SUMMARY

The present invention relates to a chemically modified fiber which hasgood absorbent properties. The fiber comprises, chemically bondedtogether, (a) a cellulosic fiber, very preferably a Kraft orchemithermomechanical fiber; (b) a poly(acrylate-co-itaconate)preferably having a relatively high acrylate content and a relativelylow itaconate content; and (c) a polyol, very preferably a polyethyleneglycol.

In more detail, the invention encompasses a chemically modified fiberhaving a water absorbency and retention value in the range from about 15g/g to about 130 g/g comprising, chemically bonded together: (a) acellulosic fiber selected from the group consisting ofchemithermomechanical pulp fiber, bleached hardwood Draft pulp fiber,bleached softwood Kraft pulp fiber, unbleached hardwood Kraft pulpfiber, unbleached softwood Kraft pulp fiber, bleached softwood sulfitepulp fiber, unbleached softwood sulfite pulp fiber, cotton linters,mercerized dissolving pulp fiber, unmercerized dissolving pulp fiber,and mixtures thereof; (b) a poly(acrylate-co-itaconate) having a weightaverage molecular weight in the range from about 60,000 to about1,000,000, an acrylate content of from about 50 mole % to about 99 mole% and an itaconate content of from about 1 mole % to about 50 mole %,and (c) a polyol; wherein the proportion by weight of saidpoly(acrylate-co-itaconate) to polyol is from about 250:1 to about 3:1and the weight of said poly(acrylate-co-itaconate) plus said polyol perunit weight of said cellulosic fiber, (a), is in the range from about0.3 to about 2, the poly(acrylate-co-itaconate) weight being expressedon an acid equivalent basis.

In the above, a preferred polyol has formula HO(CH₂ CH₂ O)_(n) H whereinn is from about 4 to about 154 and a preferred proportion by weight ofpoly(acrylate-co-itaconate) to polyol is from about 30:1 to about 4:1.

In a highly preferred embodiment, the invention provides a chemicallymodified fiber wherein said poly(acrylate-co-itaconate) has acrylatecontent of from about 90 mole % to about 95 mole % and itaconate contentof from about 5 mole % to about 10 mole %; said weight average molecularweight is in the range from about 600,000 to about 900,000; n is saidformula is from about 34 to about 100; and said weight ofpoly(acrylate-co-itaconate) plus polyol per unit weight of saidcellulosic fiber, (a), is in the range from about 0.6 to about 1.5.

In other absorbent, quick-wicking chemically modified fiber embodiments,n in said formula is from about 70 to about 80; said proportion byweight of poly(acrylate-co-itaconate) to polyol is from about 10:1 toabout 5:1; and said weight of poly(acrylate-co-itaconate) plus polyolper unit weight of cellulosic fiber, (a), is in the range from about 0.8to about 1.2. Such fiber of the invention is especially useful when saidwater absorbency and retention value is in the range from about 50 g/gto about 90 g/g.

In the above-identified fiber of the invention, cations, which areinherently present in a charge-balancing amount, are generally selectedfrom the group consisting of sodium, potassium, lithium, hydrogen andmixtures thereof, more preferably sodium, hydrogen and mixtures thereof.

A highly preferred form of the fiber for absorbency purposes is thesodium salt form, however the acid form is also useful, inter-aliabecause it can readily be taken to the absorbent form by sodiumhydroxide.

The invention encompasses papermaking pulps especially useful forwet-laying (although the same pulps are also useful in air-layingapplications). Cellulosic papermaking pulps in accordance with theinvention consist essentially of the above-identified fiber, or can be amixture of the fiber of the invention with an unmodified fiber, such asthe unmodified component (a) fiber identified supra. One such pulpconsists essentially of: from about 5% to about 60% of the chemicallymodified fiber of the invention and from about 40% to about 95% ofconventional cellulosic fiber.

Preferred chemically modified pulps in accordance with the invention areuseful in the acid form, for example the pulp is largely acid-form whenthe content of cations which are hydrogen is such as to produce a pH ofless than 5 when the pulp is dispersed in water. In this instance, theconsistency can vary widely and the pulp can be shipped at highconsistency since, as noted above, the chemically modified fiber is notabsorbent.

The invention also encompasses the absorbent form of the cellulosicpapermaking pulp, for example one comprising a major proportion ofsodium-form fiber of the invention: typically, in such a pulp, thecontent of cations which are hydrogen is such as to produce a pH ofabout 6 to about 9 when dispersed in water.

The fiber of the invention is a lightly crosslinked material. Particularattention is paid herein to adjusting the relative proportions of thestarting-material components and to process conditions so that a lightlycrosslinked fiber can best be achieved.

Thus a preferred fiber of the invention can be secured by heating forspecific curing times at particular curing temperatures a conventionalcellulosic fiber onto which has been deposited an intimate mixture ofthe poly(acrylate-co-itaconate) and polyol. For proper control of thecrosslinking, it is critical that the copolymer starting-material becapable of forming anhydride functionality during the thermal cure. Inthe case of the poly(arcylate-co-itaconate), the 1,4-diacid functional(present in the copolymer by virtue of itaconate) will dehydrate duringheating, to afford the requisite anhydride. Best results can be achievedby operating in specific, acidic pH ranges, and by control of the cationcomposition, especially by avoiding strongly co-ordinating polyvalentmetal ions.

The term "fiber" is used to distinguish the immediate product of theinvention from strong interbonded masses of paper fibers. The lattermight seem similar based on a mere recital of ingredients, but do nothave the dispersability and absorbency properties of the invention.

Thus, as noted, the fiber of the invention can be used on large scale asa papermaking pulp, especially in admixture with conventionalpapermaking fiber. Paper webs made by wet-laying such pulps areespecially useful for making disposable absorbent paper towels having aunique distribution of absorbent material, which are capable of quicklyabsorbing appreciable amounts of water or watery fluids.

Useful embodiments of the invention include a wet-laid paper web,comprising at least 1% (preferably 5% to 10%) up to about 60% ofchemically modified fiber of the invention. Excellent webs are securedwhen the content of fiber of the invention is from about 20% to about50%.

The invention also encompasses a disposable absorbent article, such as adisposable absorbent towel or a pad for a catamenial, comprising one ormore plies of a wet-laid paper web as described herein.

The invention has several significant advantages. Thus the fiber of theinvention leads to wet-laid absorbent paper free from aestheticnegatives in use, such as a tendency to shed particles of absorbentmaterial or such as a tendency to feel slippery and gel-like whenwetted. Other advantages include, but are not limited to: simplicity;nonreliance on expensive or toxic metal catalysts during thepreparation; the ability to improve the absorbency of "difficult" fiberssuch as chemithermomechanical pulp fibers (which is very desirable inview of the environmental advantages of such fibers as compared withchemical pulp fibers); and importantly, the ability to provide improvedabsorbent fibers which better accommodate the stresses of wet-layingpapermaking with less tendency to disintegrate or cause wire-blocking ordrying difficulties than conventional absorbent polymer-treated fibers.

All percentages herein are by weight and temperatures are ambient (25°C.), unless otherwise specifically noted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the important types of chemical bonding betweencellulose, acrylate, itaconate and polyethylene glycol in a fiberaccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION Chemical StructureFeatures--Description of the Drawing

A fiber in accordance with the instant invention is effectively acellulosic fiber of natural origin to which is chemically attached alightly crosslinked mixture of particular synthetic components, namely apoly(acrylate-co-itaconate) and a polyol. Without being limited bytheory, the essential features of the chemical bonding occurring in apreferred embodiment of the invention are illustrated in FIG. 1.

FIG. 1 shows, covalently bonded together, (i) a cellulose moiety, (ii)itaconate moieties (these form junctions between the other moieties),(iii) polyacrylate moieties and (iv) a polyol moiety, which in FIG. 1 isone derived from a polyethylene glycol. Not shown are the fiber as awhole, of which the depicted cellulose moiety is but a part, as well ascations (sodium being preferred), which are inherently present in acharge-balancing amount and are primarily associated with the negativecharges of the copolymer; water molecules; and any imperfections, suchas incompletely reacted moieties.

Important features of the invention illustrated in FIG. 1 include thatthe relative proportion of itaconate to acrylate is low, ensuring arelatively light crosslinking. Moreover, the lightly crosslinkedacrylate/itaconate/polyol structure is chemically attached to thecellulosic fiber.

The fibers of the invention are quite different from grafted cellulosicfibers of well-known kinds which can be made, for example bycerium-catalyzed polymerization of methyl acrylate in presence of acellulosic fiber followed by sodium hydroxide hydrolysis. A knowntechnique for finding the location of a highly charged syntheticpolycarboxylate polymer in relation to a material such as cellulosewhich is less highly charged involves mapping the distribution ofcharge-balancing cations, such as sodium, by X-ray Energy DispersiveSpectroscopy. Sodium ion maps of preferred fibers of the invention showsubstantially intact cellulose regions without high concentrations ofsodium being present between the fiber wall and the fiber lumen, whereasthere is a significant proportion of sodium distributed between thefiber wall and the lumen in typical hydrolyzed methyl-acrylate graftedfibers.

Without being bound by theory, it is believed that having a fiber withsubstantially intact cellulosic regions of natural origin and achemically attached, lightly crosslinked, water-swellablepoly(acrylate-co-itaconate are important for securing the benefits ofthe instant invention.

Composition of fiber of the invention

In general, the fiber of the invention comprises, chemically bondedtogether: (a) a cellulosic fiber; (b) a poly(acrylate-co-itaconate and(c) a polyol.

(a). Cellulosic Fiber.

The cellulosic fiber, to which the remaining components are bonded inthe fiber of the invention, is a conventional material. In general, itis selected from the group consisting of chemithermomechanical pulpfiber, bleached hardwood Kraft pulp fiber, bleached softwood Kraft pulpfiber, unbleached hardwood Kraft pulp fiber, unbleached softwood Kraftpulp fiber, bleached softwood sulfite pulp fiber, unbleached softwoodsulfite pulp fiber, cotton linters, mercerized dissolving pulp fiber,unmercerized dissolving pulp fiber, and mixtures thereof.

Preferred cellulosic fiber is selected from the group consisting ofchemithermomechanical pulp fiber, bleached hardwood Kraft pulp fiber,bleached softwood Kraft pulp fiber, unbleached hardwood Kraft pulpfiber, unbleached softwood Kraft pulp fiber, and mixtures thereof.

Highly preferred embodiments of the invention include those made fromchemithermomechanical pulp fiber and bleached Kraft fiber such assouthern softwood Kraft fiber. As will be seen hereinafter, there aresomewhat different preferred synthesis conditions, especially relatingto pH, curing temperature and curing time, depending on which of thesehighly preferred fibers is chosen.

Preferably, such fiber will be of a quality deemed good or superior forthe purposes of conventionally making wet-laid paper webs. Morespecifically, fiber having relatively low levels of "fines " and a goodstaple length are preferred.

(b). Poly(acrylate-co-itaconate).

The fiber of the invention also contains a poly(acrylate-co-itaconate).In general, this copolymer has an acrylate content of from about 50 mole% to about 99 mole %, more preferably about 70 mole % to about 98 mole%, most preferably about 90 mole % to about 95 mole % and an itaconatecontent of about 1 mole % to about 50 mole %, more preferably about 2mole % to about 30 mole %, most preferably about 5 mole % to about 10mole %.

The poly(acrylate-co-itaconate) useful herein are selected members of aknown class of copolymers. They may be prepared by conventional,art-known techniques for copolymerizing acrylic acid and itaconic acid.Typically, mild aqueous conditions using a conventional water-solublefree-radical initiator are used. Suitable initiators are illustrated bythe common azo initiators such as 2,2'-azobis(2-amidinopropane)dihydrochloride; as well as potassium persulfate, hydrogen peroxide orthe like. Selection of a copolymer useful herein is based on arecognition that any given itaconate moiety in the copolymer will beunreactive for the purposes of forming the fiber of the invention if atthe time thermal crosslinking is attempted, an itaconic anhydride moietycannot be formed. Such resistance to forming the anhydride is exhibitedespecially when the itaconate component of the copolymer is completelyneutralized, such as in the form of the diammonium salt, and to an evengreater extent, when the itaconate component of the copolymer isco-ordinated with multivalent metal ions such as those of calcium,magnesium or iron. For practical purposes, it is therefore highlypreferred both to use the acid form of the copolymer and to limit thecontent of multivalent metal cations in the copolymer. The latter canbest be achieved by synthesizing the copolymer in clean water.

Although isolable, the copolymer can conveniently be made, and furtherdirectly used to form the fiber herein, as an aqueous solution.

Copolymers most useful herein have weight average molecular weight,M_(W), as determined by low angle laser light scattering, in the generalrange from about 60,000 to about 1,000,000. Preferred copolymer hasweight average molecular weight of from about 400,000 to about1,000,000. Within practical limits, the absorbent properties of thefiber of the invention increase significantly as the weight averagemolecular weight of the copolymer increases. Since copolymers tend tobecome viscous and difficult to handle at very high weight averagemolecular weight, a highly preferred copolymer weight average molecularweight is in the range from about 600,000 to about 900,000.

As found in the fiber of the invention, the copolymer is chemicallybonded to the cellulosic fiber and to the polyol.

(c). Polyol

The polyol component of the fiber of the invention is an alcohol havingtwo or more --OH groups. In the fiber of the invention, as illustratedin FIG. 1, the polyol is at least partially chemically incorporated byreaction with itaconic anhydride moieties of the copolymer (see thechemistry of the synthesis further discussed hereinafter) so that it isno longer in the free state and acts as a crosslinking group in thefiber of the invention. Although a wide variety of polyols are usefulherein, preferred polyols are water-soluble. Although any polyolconsisting essentially of C, H and O can be used, the polyol istypically selected form the group consisting of polyethylene glycol,polyvinyl alcohol, ethylene glycol, propylene glycol, glycerin,pentaerythritol and the like.

Another polyol capable of being substituted for propylene glycol withinthe spirit and scope of the invention is a relatively longer chainalpha-omega alkylene diol, such as a 1,6 hexylene glycol.

In preferred embodiments, the polyol is a diol, such as polyethyleneglycol, and can have varying molecular weight. Suitable diol materialshave the formula HO(CH₂ CH₂ O)_(n) H wherein n is from about 4 to about154, more preferably from about 34 to about 100, most preferably fromabout 70 to about 80. Preferred embodiments of these materials are to befound in the commercial PEG 200-7000 series. Thus, commercial PEG 200corresponds with n in the above formula of about 4, PEG 1000 correspondswith n of about 22, PEG 1500 corresponds with n of about 34, PEG 3350corresponds with n of about 76 and PEG 6800 corresponds with n of about154. In practice, it is found that although quick-wicking fiber of theinvention can be prepared with PEG 200, absorbency results are optimalwith PEG 3350. PEG 6800, though usable, gives somewhat less preferredembodiments of the invention.

Proportions of components

In general, the proportion by weight of poly(acrylate-co-itaconate) topolyol in the fiber of the invention is in the range from about 250:1 toabout 3:1, more preferably from about 30:1 to about 4:1, most preferablyfrom about 10:1 to about 5:1. When the polyol has a low molecularweight, such as ethylene glycol, the weight amount of polyol isrelatively low. As the molecular weight increases in a homologousseries, such as in PEG of progressively increasing molecular weight, therelative weight of polyol increases. When the polyol has many --OHgroups, as in polyvinylalcohols, the relative proportion by weight ofpolyol may be low, even though the molecular weight of the polyol ishigh.

Without being limited by theory, it is believed that the proportion ofcopolymer to polyol is very important for controlling the crosslinkdensity in the fiber of the invention. The above-recited ranges takeinto account that maintaining a relatively constant, consistently lowcrosslink density is preferred.

In the fiber of the invention, the add-on, that is to say the weight ofpoly(acrylate-co-itaconate) plus polyol ((b) plus (c)) per unit weightcellulosic fiber (a) is in the general range from about 0.3 grams pergram to about 2 grams per gram, more preferably from about 0.6 grams pergram to about 1.5 grams per gram, most preferably from about 0.8 toabout 1.2 grams per gram. It should be appreciated that as the add-on isprogressively decreased, the absorbency of the fiber decreases but thefiber may become somewhat easier to process. On the other hand,excessive add-on, outside the scope of this invention, can lead to anappreciable content of pieces of absorbent polymer which are notchemically attached to the fiber. Moreover, there appears to be aplateau effect of absorbency performance and usefulness when much morethan the stated upper limit of add-on is used.

It should be appreciated that add-on levels herein are, in percentageterms, rather high, i.e., 30% to 200%. These levels are very much higherthan in conventional paper coatings, wet-strength additive applicationsor the like.

The poly(acrylate-co-itaconate) weight referred to hereinabove andthroughout the specification is by convention expressed on an acidequivalent basis. That is to say, regardless of the form of thepoly(acrylate-co-itaconate) used in the synthesis of the fiber of theinvention, and equally regardless of the form product fiber, theconvention is adopted of everywhere specifying thepoly(acrylate-co-itaconate) weight as though it were in the acid form,i.e., all the charge-balancing cations are H. In this manner, therelative proportion of poly(acrylate-co-itaconate) to the cellulose andpolyol components is unambiguously determined.

Cations

Since the fiber of the invention contains negatively charged carboxylategroups, especially those associated with the poly(acrylate-co-itaconate)cations will inherently be present in a charge-balancing amount.

In the fiber of the invention, the cations are generally selected fromsodium, potassium, lithium, hydrogen and mixtures thereof, morepreferably sodium, potassium, hydrogen and mixtures thereof, mostpreferably sodium, hydrogen and mixtures thereof.

A similar range of cation composition applies to the copolymerstarting-material, however the most highly preferred cation for thestarting-material copolymer is hydrogen. Thus the starting form of thecopolymer is most preferably the acid form.

The fiber of the invention can be in the acid form, in which it is notdirectly useful as in absorbent material but is very useful forlong-term storage or shipping from the fiber manufacturing plant to thepapermaking plant at high consistency; or it can be int he highlyabsorbent sodium form. Other such water-soluble monovalent cation saltsof the fiber of the invention, such as the potassium salt, as noted, arelikewise useful absorbents.

Importantly, polyvalent cations such as those of iron, calcium,magnesium and aluminum are avoided, both in the starting copolymer andin the fiber of the invention, as much as practical considerations willallow. Such cations can not only interfere with the synthesis of thefiber but also with the absorbent properties of the product fiber.

Absorbency property

The fiber of the invention is most useful as an absorbent material.Thus, it has a water absorbency and retention value (WAARV)--thisquantity being measured according to the procedure given in "TestMethods" hereinafter--in the range from about 15 g/g to about 130 g/g,more preferably from about 30 g/g to about 100 g/g, most preferably fromabout 50 g/g to about 90 g/g.

The term "retention" in WAARV takes into consideration that the testmethod involves centrifugation, so that water quite tenaciously retainedby fiber, pulp or paper is included in the absorbency measurement.Moreover, WAARV values are measured at a constant alkaline pH so thatvalues are reproducible and can be compared. WAARV can be used tocharacterize both acid-form and salt-form fibers according to theinvention because during the test, in-situ conversion of acid-form tosalt-form fiber takes place. Moreover, WAARV can be used to measure theabsorbency of wet-laid webs comprising the fiber of the invention.

Without being bound by theory, it is believed to be important that thefiber herein is substantially discrete rather than a mass of stronglyinterbonded fibers with significant amounts of polymer located at thefiber crossovers. That latter behavior, believed inferior for absorbencypurposes, is the kind to be expected when the fiber results from (i)forming paper, e.g., on a Fourdrinier wire then (ii) applying a polymer,for example by spray-on or impregnation, then (iii) crosslinking thepolymer: such a sequence is not in accordance with this invention.

Thus the fiber of the instant invention results from (i) concentratingthe polymer components as much as possible on individual cellulosicfibers prior to making paper, then (ii) thermally crosslinking to formthe chemical bonds between the polyol-copolymer mixture and the fiber.The resulting fiber can then (iii) be used in bulk as a papermaking pulpor furnish for wet-laying to achieve an absorbent, quick-wicking paperweb free from aesthetic disadvantages.

Chemistry of Synthesis

It should be understood that water (H₂ O) is eliminated in the chemicalreactions of curing or thermally crosslinking which are normally used toform the fiber of the invention. Without being limited by theory, thefollowing chemical reactions are believed to occur:

(I) all or at least part of the itaconate moieties of the copolymerdehydrate in the presence of heat to give itaconic anhydride moieties;

(II) a portion of the itaconic anhydride moieties further react byacylating the --OH groups of the cellulosic fiber, (a): this results inat least partial chemical attachment of copolymer to fiber via covalentester bonds of cellulose to itaconate; and

(III) a portion of the itaconic anhydride moieties further react byacylating the --OH groups of the polyol: since the polyol is at leastdifunctional, this results in crosslinking of the copolymer and polyol.

Although it is believed that substantially all itaconate moieties areaccounted for by participating in reaction (II) or (III), it is normalpractice herein to provide a slight excess of itaconate moieties beyondthat required for complete reaction. Thus the fiber of the invention maycontain traces of non-crosslinked itaconate and, although unlikely, itis believed that traces of anhydride-form itaconate may be present indry fiber of the invention.

To be noted is that the terms "curing", "thermally crosslinking","crosslinking" and "chemically reacting" are equivalent herein, at leastinasmuch as they refer more or less specifically to producing the fiberof the invention. Curing temperatures and times are very important andare discussed at length hereinafter.

Preparation of fiber of the invention

In general, fiber of the invention can be made by lightly crosslinking,typically by a thermal method, a cellulosic fiber of a quality suitablefor wet-laying papermaking, typically a conventional wet-layingpapermaking fiber such as bleached southern softwood Kraft orchemithermomechanical fiber, with an intimate mixture ofpoly(acrylate-co-itaconate) and polyol.

It is essential that immediately prior to thermal crosslinking, thepoly(acrylate-co-itaconate) should be capable of producing an acidicsolution in water, for the simple reason that the fully neutralizedsalts, such as the diammonium salt, the disodium salt etc., areincapable of thermally eliminating water and of forming an anhydride,which as discussed supra, is an essential part of the chemical reactionleading to the fiber of the invention.

It is consistent with the sense of the invention to depositcopolymer-polyol mixtures on cellulosic fibers using a process such asextrusion, evaporative deposition or any similar deposition method,regardless of whether it involves a fluid medium or carrier or not.

When there is no medium to be removed, the mixed fiber/copolymer/polyolcan be directly crosslinked by heating at suitable curing temperaturesfor limited curing times, always provided that a suitably intimatemixture has been formed.

When it is desired to make fibers without resorting to expensive processequipment, a medium can be used to deposit thepoly(acrylate-co-itaconate) on the starting-material cellulosic fiber.In this event, the medium should preferably be capable of substantiallycompletely dissolving the poly(acrylate-co-itaconate) and the polyol, sothat an intimate mixture of the two can be evaporatively deposited onthe cellulosic fiber. The medium should be capable of substantiallycomplete evaporation at normal or reduced pressures below thetemperatures at which thermal crosslinking occurs. Acetone/watermixtures, acetone/water/methanol, mixtures, and methanol/water mixturesare all quite suitable, as are water mixtures with other commonlow-boiling water-miscible organic solvents, but water alone is highlypreferred, especially on account of low cost and low toxicity.

When water dissolves the poly(acrylate-co-itaconate) and polyol, theresult is an "aqueous medium" for the purposes of this invention.Typically, the aqueous medium has a percentage by weight ofpoly(acrylate-co-itaconate) plus polyol which is about 10% by weight orhigher, more preferably the concentration is about 20%. Much moreimportantly, the aqueous medium is found to behave quite differently, interms of its suitability, depending on the pH. In general, the pH of theaqueous medium must lie in the range from about 1.8 to about 4.0, morepreferably from about 2.5 to about 3.5. When the cellulosic fiber to betreated is chemithermomechanical fiber, a pH range of from about 1.8 toabout 4.0 is acceptable. When other cellulosic fiber types are beingtreated, it is essential that the pH of the aqueous medium should be inthe range from about 2.5 to about 4.0. Below the above-specified pHminima, depending on the precise type of cellulosic fiber, thecellulosic fiber will tend to degrade. Moreover, at pH values much abovepH the stated upper limit, the degree of crosslinking in thecrosslinking step is sharply reduced, to an unacceptable extent.

Water used to make the aqueous medium is preferably substantially freefrom polyvalent cations such as those of calcium, magnesium, iron andaluminum. In any event, the content of such cations should not be sohigh as to inhibit the thermal crosslinking reaction.

Once the poly(acrylate-co-itaconate) and polyol are dissolved and anaqueous medium is formed, the medium can be applied to cellulosic fibersin whatever manner desired, provided that these fibers are discrete ordispersed rather than knit together in the form of a bonded web.

The mixture of cellulosic fibers and aqueous medium is evaporated atnon-crosslinking temperatures. For practical purposes, such temperaturesare generally below about 75° C., typically in the range 50° C. to 70°C. At higher temperatures, there is an increased risk of uncontrolledcrosslinking. Lower temperatures can be used: for example water can beevaporated by freeze-drying.

Evaporation of water results in a substantially dry, intimate mixture ofthe poly(acrylate-co-itaconate) and polyol on the cellulosic fibers.

Preferably, the evaporation is carried out under conditions which avoidsticking together of the fibers. One suitable approach believed to begood for removing water from the aqueous medium and uniformly depositionthe poly(acrylate-co-itaconate) and polyol as an intimate mixture on thecellulosic fiber surface involves the use of a supercritical fluid suchas supercritical carbon dioxide for extracting the water.

A preferred approach to the depositing operation which has been foundquite satisfactory, especially on grounds of economy and simplicity, isto evaporate a thin layer of thepoly(acrylate-co-itaconate)/polyol/fiber/water mixture. Although theremay be some sticking together of fibers, the evaporated layer is readilyrepulped (after the crosslinking step described in detail below) to givesubstantially discrete fibers of the invention.

In general, crosslinking or "curing" herein involves applying acontrolled amount of heat, which can be achieved under a range oftemperatures and times.

Thus, in a preferred embodiment, the invention encompasses a process forpreparing a chemically modified fiber having a water absorbency andretention value in the above-recited ranges, comprising a step of:

thermally crosslinking, at a curing temperature of from about 100° C. toabout 150° C., more preferably from about 110° C. to about 140° C. for acuring time of from about 60 minutes to about 2 minutes, more preferablyfrom about 33 minutes to about 3 minutes, a starting-material pulpconsisting essentially of the above-identified cellulosic fiber(component (a)); with an intimate mixture of poly(acrylate-co-itaconate)(above-identified as (b), and a polyol (component (c) identifiedhereinabove); wherein the proportion by weight ofpoly(acrylate-co-itaconate) copolymer to polyol is in the above-recitedgeneral ranges and the weight of poly(acrylate-co-itaconate) plus polyolper unit weight of cellulosic fiber, (a), is likewise as recitedhereinabove.

In practice, the copolymer-polyol treated dry cellulosic fibers arepreferably exposed to heat as a thin layer. Preferably, a pre-heatedoven is used for best control of the crosslinking or curing reaction. Inorder to minimize curing time at any given curing temperature, thepractitioner will preferably use a flow of hot air and will permitaccess of the hot air to both sides of the fiber layer by first removingany substrate which may have been used in the above-describedfiber-polymer contacting procedures: in practice, this is most easilyaccomplished when the substrate is "non-stick", for example,polytetrafluoroethylene (PTFE).

Based on this appreciation of the curing operation, the practitionerwill readily appreciate that it is possible to conduct evaporativedeposition and thermal crosslinking, indeed the entire synthesis of thefiber of the invention, in a continuous or semi-continuous mode. Forexample, a PTFE carrier belt can carry the cellulosic fiber, copolymerand polyol through the evaporative deposition stage and into the curingstage in the synthesis process.

It is however been found that it is preferred to cure in the absence ofa substrate, or in the presence of a substrate which does not overlyaffect heat flow into the above-described thin layer during curing.

Thus, when curing a fiber/copolymer/polyol layer about 1 mm thick onglass about 3 mm thick, the layer being one resulting from evaporationof an aqueous medium having a pH of 3.00, the following curingtemperatures and the corresponding curing times are illustrative ofpreferred curing conditions:

    ______________________________________                                        Curing Temperature (°C.)                                                                Curing Time (minutes)                                        ______________________________________                                        110              33                                                           120              18                                                           130                11.5                                                       140               8                                                           ______________________________________                                    

In the above and throughout the specification, curing times are definedas the total period of exposure to hot air at the curing temperature,the fiber layer being introduced to the hot air oven at ambienttemperature.

Substituting PTFE for glass as a substrate for evaporating the aqueousmedium in the above, and removing the PTFE from a layer now about 2 mmthick prior to curing, a preferred curing time at a temperature of 130°C. is reduced from 11.5 minutes (glass-see the above) to about 6.5minutes (no substrate). The reduction in the curing time is believed tobe due to the improved access of hot air to both sides of the fibrouslayer.

In light of the above, the practitioner should be aware that for bestresults, especially when manufacturing on a large scale, it is advisableto optimize the curing temperature and time at the scale chosen, by thesimple expedient of measuring Fiber Yield and water absorbency andretention values of the fiber of the invention, each as defined in "TestMethods" hereinafter, over a series of curing temperatures and times inaccordance with the invention.

Once curing is complete, the raw fiber of the invention is repulped,preferably with an amount of shear which will not significantly reducethe staple. Repulping is generally carried out in water under acidconditions, typically at a pH of about 2 to about 4, more preferably atpH of about 2 to about 3 (hence the term "acid repulping" can be used tocharacterize this step). In the acid repulping step, the fiber of theinvention is substantially in the acid form. In this form, the fiber isnon-swollen and is readily manipulated, thus in this form it has theadvantage that it can conveniently be shipped as a concentrated slurryfrom the fiber manufacturing plant to the papermaking plant if desired.

After repulping, the fiber of the invention can be secured substantiallyin the dry, sodium-salt form by a fiber-swelling step. Thefiber-swelling step simply involves neutralizing with sodium hydroxide,preferably to a pH of from about 7.5 to about 9, whereon the fiberswells greatly. The fiber swelling step can be quite slow, and may takeup to 2-3 days. It is a curious feature of the fiber of the inventionthat the first conversion from the acid form to the sodium salt form isof such duration, since subsequent interconversions between the acid andsalt forms can be carried out quite rapidly by adding acid or base, asneeded.

If desired, after the fiber-swelling step, the fiber of the inventioncan be filtered and dried, typically at temperatures of about 80° C.-90°C., although this is not necessary and is not usually practiced if thefibers are to be used as a pulp for wet-laying papermaking.

To be noted is that fibers of the invention in the sodium salt form aresuperior in their heat resistance as compared with the correspondingfibers in the acid form. If in the above, the sodium hydroxide issubstituted by potassium hydroxide or lithium hydroxide, thecorresponding potassium and lithium salt forms of the fiber of theinvention can be secured.

For the practical reason that the fiber of the invention is typicallyused in a wet-laying process, the practitioner generally does need todry the salt form of the fiber prior to use in wet-laying, but candirectly use it as a slurry.

Webs and Wet-Laying Processes for their Production

In other embodiments, the invention provides a wet-laid paper webcomprising from about 5% to about 60%, more preferably from about 10% toabout 60%, most preferably from about 20% to about 50% of the fiber ofthe invention (or equivalently, the product of the above-identifiedsynthesis process). The balance of the composition can be conventionalpapermaking fibers, such as fibers having an identical composition tothe starting-material fibers. When mixtures of fiber of the inventionand conventional papermaking fibers are co-distributed in a wet-laidweb, highly absorbent, quick-wicking structures result.

Preferred papermaking processes useful herein, as incorporated byreference in the background art discussion hereinabove, includecontinuous wet-laying processes designed for making conventional highlyabsorbent paper.

A feature of interest which distinguishes several such processes and isbelieved to be useful in the context of the present invention is toavoid compressing or squeezing (e.g., calendering) the wet-laid web asmuch as possible during drying: also, it can be helpful to dry the webscontaining the fiber of the invention using blow-through air dryers ofconventional construction. This produces a rather open, absorbent web.

A modification of a conventional wet-laying process which is especiallyhelpful for making wet-laid webs according to the present invention in acontinuous operation simply involves wet-laying at acidic pH, typicallyin the range from about 3 to about 5, followed by partially drying thewet-laid web, neutralizing on-line with a sprayed-on sodium carbonate orpotassium carbonate solution (sodium hydroxide may be used but canyellow the web is not carefully applied), and drying, especially withthe aid of a conventional Yankee dryer.

Disposable absorbent towels

The wet-laid webs can be used as plies in a two-ply or multi-plydisposable absorbent structure such as a disposable absorbent towel. Allthat needs to be done to secure such disposable absorbent structures isto combine plies comprising at least one wet-laid paper web according tothe invention, in a conventional converting operation, e.g., simpleglueing or bonding of the plies together.

ALTERNATE EMBODIMENTS OF THE INVENTION

The fiber of the invention is not limited to use as an absorbent fordisposable absorbent towels, but may be used for making catamenial pads,absorbent dressings, pantiliners and the like.

Fibers in accordance with the invention are further illustrated by thefollowing Examples.

EXPERIMENTAL Starting-materials

Acrylic acid (Polysciences Inc., Warrington, Pa.) is vacuum distilledthrough a Vigreux column and is preferably used fresh in subsequentoperations, e.g., within one day of distillation. Itaconic acid (AldrichChemical Co., Milwaukee, Wis.) is obtained in 99%+purity and is used asreceived. The free-radical initiator 2,2'-azobis(2-amidinopropane)dihydrochloride (WAKO V-50, Wako Pure Chemical Industries, Osaka, Japan)is also used as received. Unless otherwise noted, water is triplydistilled. Where polymers are dialyzed, the dialysis membrane isobtained from Spectrum Medical Industries, Inc., Los Angeles, Calif.

Polyethylene glycols (these preferred polyols are commonly known as"PEG", various suppliers being suitable) as used in the Examples havenominal molecular weights of 200, 1000, 1500, 3350, and 6800. PEG 200 isobtained from Polysciences Inc., Warrington, Pa. PEG 1000, PEG 1500 andPEG 6800 are obtained from Scientific Polymer Products, Inc., Ontario,N.Y. PEG 3350 is obtained from Sigma Chemical Co., St. Louis, Mo.

Southern softwood Kraft pulp and northern softwood Kraft pulp areobtained from P & G Cellulose, Memphis, Tenn. Chemithermomechanical pulpis obtained from Quesnel Paper Co., Quesnel, B.C., Canada.

EXAMPLE I Preparation of an poly(acrylate-co-itaconate) suitable for usein making fiber of the invention (90 mole % acrylate, 10 mole %itaconate)

Acrylic acid (20.000 g, 0.27755 mole), itaconic acid (4.0121 g, 0.038386mole), Wako V-50 (0.0837 g, 0.308 millimole), and 150 ml of water whichhas been acidified to pH 2.0 with hydrochloric acid are added to a 250ml three-necked round-bottomed flask. The necks are fitted with athermometer, a stopper, and a gas inlet/outlet adapter capable ofbubbling gas through a liquid in the flask and venting it. The solutionis deaerated by passage of nitrogen gas and is then placed under anatmosphere of argon. The solution is heated to 55° C. and is maintainedat this temperature for 15 hours. The viscous solution of copolymer iscooled to ambient temperature and is dialyzed overnight against water(Spectrapor 3 tubing with molecular weight cut-off at 3500) to removeany unreacted monomers. The dialyzed solution is freeze dried to afford23.00 g of poly(acrylate-co-itaconate), acid form, as a colorless solid.The weight average molecular weight, M_(W), as determined by low anglelaser light scattering in 0.2 Molar sodium chloride in water (refractiveindex=1.3344, d_(n) /d_(c) =0.1683) is 896,100.

EXAMPLE II Preparation of another poly(acrylate-co-itaconate) suitablefor making fiber of the invention (90 mole % acrylate, 10 mole %itaconate)

Acrylic acid (25.000 g, 0.34693 mole), itaconic acid (5.0151 g, 0.038548mole), Wako V-50 (0.1046 g, 0.3856 millimole), and 193 ml of water whichhas been acidified to pH 2.0 with hydrochloric acid are added to a 500ml three-necked round-bottomed flask. The necks are fitted with athermometer, a stopper, and a gas inlet/outlet adapter capable ofbubbling gas through a liquid in the flask and venting it. The solutionis deaerated by passage of nitrogen gas and is then placed under anatmosphere of argon. The solution is heated to 60° C. and is maintainedat this temperature for 15 hours. The viscous solution of copolymer iscooled to ambient temperature and is dialyzed against distilled waterovernight (Spectrapor 3 tubing as in the foregoing Example) to removeany unreacted monomers. The dialyzed solution is freeze dried to afford28.31 g of poly(acrylate-co-itaconate), acid form, as a colorless solid.The weight average molecular weight, M_(W), as determined by low anglelaser light scattering in 0.2 Molar sodium chloride in water (refractiveindex=1.3344, d_(n) /d_(c) =0.1683) is 658,200.

EXAMPLE III Preparation of another poly(acrylate-co-itaconate) suitablefor making fiber of the invention (90 mole % acrylate, 10 mole %itaconate)

Acrylic acid (105.27 g, 1.4609 moles), itaconic acid (21.12 g, 0.1623mole), Wako V-50 (0.4403 g, 1.623 millimole), and 812 ml of water whichhas been acidified to pH 2.0 with hydrochloric acid are added to a 2liter three-necked round-bottomed flask. The necks are fitted with athermometer, a stopper, and a gas inlet/outlet adapter capable ofbubbling gas through a liquid in the flask and venting it. The solutionis deaerated by passage of nitrogen gas and is then placed under anatmosphere of argon. The solution is heated to 55° C. and maintained atthis temperature for 15 hours. The viscous solution of copolymer iscooled to ambient temperature, and is freeze dried to give 121.57 g ofpoly(acrylate-co-itaconate), acid form, as a colorless solid. The weightaverage molecular weight, M_(W), as determined by low angle laser lightscattering on a dialyzed portion in 0.2 Molar sodium chloride in water(refractive index=1.3344, d_(n) /d_(c) =0.1683) is 821,600.

EXAMPLE IV Preparation of another poly(acrylate-co-itaconate) suitablefor making fiber of the invention (90 mole % acrylate, 10 mole %itaconate)

Acrylic acid (1050.0 g, 14.571 moles), itaconic acid (210.64 g, 1.6190moles), Wako V-50 (4.3919 g, 16.19 millimole), and 7.9 liters of waterwhich has been acidified to pH 2.0 with hydrochloric acid are added to a22 liter three-necked round-bottomed flask fitted with a thermometer, amechanical stirrer, and gas inlet/outlet adapter capable of bubbling gasthrough a liquid in the flask and venting it. The solution is deaeratedby passage of nitrogen gas and is then placed under an atmosphere ofnitrogen. The solution is heated to 55° C. and maintained at thistemperature for 15 hours. The viscous solution of copolymer is cooled toambient temperature and is freeze dried to give 1,222.1 g ofpoly(acrylate-co-itaconate), acid form, as a colorless solid. The weightaverage molecular weight, M_(W), as determined by low angle laser lightscattering on a dialyzed portion in 0.2 Molar sodium chloride in water(refractive index=1.3344, d_(n) /d_(c) =0.1683) is 711,700.

EXAMPLE V Preparation of another poly(acrylate-co-itaconate) suitablefor making fiber of the invention (95 mole % acrylate, 5 mole %itaconate)

Acrylic acid (25.00 g, 0.3469 mole), itaconic acid (2.376 g, 18.27millimole), Wako V-50 (0.0991 g, 0.365 millimole), and 183 ml of waterwhich as been acidified to pH 2.0 with hydrochloric acid are added to a500 ml three-necked round-bottomed flask. The necks are fitted with athermometer, a stopper, and a gas inlet/outlet adapter capable ofbubbling gas through a liquid in the flask and venting it. The solutionis deaerated by passage of nitrogen gas and is then placed under anatmosphere of argon. The solution is heated to 55° C. and maintained atthis temperature for 15 hours. The viscous solution of copolymer iscooled to ambient temperature and is dialyzed against distilled waterovernight (Spectrapor 3 tubing as in the foregoing Examples) to removeany unreacted monomers. The dialyzed solution is freeze dried to afford25.99 g of poly(acrylate-co-itaconate), acid form, as colorless solid.The weight average molecular weight, M_(W), as determined by low anglelaser light scattering in 0.2 Molar sodium chloride in water (refractiveindex=1.3344, d_(n) /d_(c) =0.1683) is 683,900.

EXAMPLE VI Preparation of another poly(acrylate-co-itaconate) copolymersuitable for making fiber of the invention (95 mole % acrylate, 5 mole %itaconate)

Acrylic acid (21.11 g, 0.2930 mole), itaconic acid (2.0061 g, 15.420millimole), Wako V-50 (0.0837 g, 0.309 millimole), and 150 ml of waterwhich has been acidified to pH 2.0 with hydrochloric acid are added to a250 ml three-necked round-bottomed flask. The necks are fitted with athermometer, a stopper, and a gas inlet/outlet adapter capable ofbubbling gas through a liquid in the flask and venting it. The solutionis deaerated by passage of nitrogen gas and is then placed under anatmosphere of argon. The solution is heated to 55° C. and maintained atthis temperature for 15 hours. The viscous solution of copolymer iscooled to ambient temperature and a portion is dialyzed againstdistilled water overnight (Spectrapor 3 tubing as in the foregoingExamples) to remove any unreacted monomers and then freeze dried toafford 1.5 g of poly(acrylate-co-itaconate), acid form, as a colorlesssolid. The remainder of the solution is diluted with water to give a 12%solids content and is used directly, without drying, in the synthesis offibers in accordance with the invention. The weight average molecularweight, M_(W), as determined by low angle laser light scattering on thedialyzed portion in 0.2 Molar sodium chloride in water (refractiveindex=1.3344, d_(n) /d_(c) =0.1670) is 925,000.

EXAMPLE VII Preparation of fiber of the invention

Poly(arcylate-co-itaconate) of EXAMPLE III (2.00 g) is dissolved byadding it portionwise to 20 ml of water while stirring and heating to65°-70° C. To the solution is added polyethylene glycol (0.334 g,nominal molecular weight 3350) predissolved in 5 ml of water. Stirringis continued until dissolution is complete. The resulting aqueous mediumis cooled to ambient temperature and the pH is adjusted to 3.00 (the "pHof the aqueous medium" referred to elsewhere herein) with 1 Molar sodiumhydroxide. Loose fibers of southern softwood Kraft pulp (2.00 g bone-dryweight basis) are added. The resulting slurry is thoroughly mixed and isspread out into a thin layer on a 6-inch diameter watch glass ofthickness about 3 mm. The slurry layer is dried in an oven at 65°-70°C., a temperature selected to minimize or avoid crosslinking reactions,and is then cured by placing the watch glass in an oven preheated to acuring temperature of 130° C. The curing time is 11.5 minutes. Thelayer, now about 1 mm thick, is cooled to ambient temperature. Thisyields fiber in the acid form, which is not particularly absorbent. Thefiber is then repulped. In practice it is convenient to soak it withdistilled water, tear it into small pieces and add it to 400 ml ofdistilled water. After further stirring (e.g., overnight) the pH of themixture is adjusted to 2.0 with hydrochloric acid and it is mixed in aWaring Blendor in two steps wherein (1) the blendor is run on low speedfor 5.0 minutes at 50% power and (2) the blendor is run for 1.0 minuteon low speed at full power. The fibers, still in the acid form, arecollected by suction filtration in a Buchner funnel fitted with ahandsheet forming wire, washed the 400 ml of water, and are re-suspendedinto 500 ml of water. The slurry pH is adjusted to 8.5 using 1 Molarsodium hydroxide in water. (Using potassium hydroxide or lithiumhydroxide instead of sodium hydroxide at this stage would result in thepotassium or lithium form of the fibers.) Over two days, the pH isperiodically checked and readjusted to 8.5 with sodium hydroxide. Duringthis period, the fibers exchange to the sodium salt form, which ishighly absorbent. Thus, the fibers swell up. The fully swollen fibers ofthe invention are collected by suction filtration and are washed withdistilled water. Their wet weight is 232.62 g and their consistency(Test Method given hereinafter) is determined to be 1.656%, from whichthe Fiber Yield (Test Method given hereinafter) is calculated to be 3.85g of fiber of the invention. The Conversion (Test Method givenhereinafter) is calculated as about 89%. The WAARV of the fiber of thisExample (Test Method given hereinafter) is determined as 96.3 g/g.

EXAMPLE VIII Preparation of fiber of the invention

Poly(acrylate-co-itaconate) of EXAMPLE IV (25.00 g) is dissolved byadding it portionwise to 250 ml of water while stirring and heating to65°-70° C. To the solution is added polyethylene glycol (4.1667 g,nominal molecular weight 3350) predissolved in 15 ml of water. Stirringis continued until dissolution is complete. The resulting aqueous mediumis now cooled to ambient temperature and the pH is adjusted to 3.00 with1 Molar sodium hydroxide. Loose fibers of southern softwood Kraft pulp(25.00 g bone-dry weight basis) are added and the resulting slurry ismixed thoroughly after each portion of pulp is added. The slurry isspread out as a thin, 15-inch by 11-inch layer on a suitably sizedpolytetrafluoroethylene (TEFLON) sheet. The layer is dried in an oven at65°-70° C., a temperature selected to minimize or avoid crosslinkingreactions, and is then cured by removing it from the TEFLON (for betterair-flow) and placing it into an oven, preheated to a curing temperatureof 130° C. The curing time is 6.5 minutes. This yields a layer about 2mm thick of acid-form fiber. This is broken into small pieces and isadded to 3 liters of distilled water. After further stirring (e.g.,overnight) the pH of the mixture is adjusted to 2.0 with 6 Molarhydrochloric acid and it is mixed in a Waring Blendor in two stepswherein (1) the blendor is run on low speed for 20 minutes at 50% powerand (2) the blendor is run for 2.5 minutes on low speed at full power.The acid-form fibers are collected by suction filtration in a Buchnerfunnel fitted with a handsheet forming wire and washed with 3 liters ofdistilled water and are re-suspended in another 4 liter aliquot ofdistilled water. The slurry pH is adjusted to 6.5 using 1 Molar sodiumhydroxide in water. The fibers exchange sodium for hydrogen, at leastsufficiently to be absorbent. The fibers swell up relatively quickly ascompared with Example VII. The pH is periodically re-adjusted to 6.5with sodium hydroxide over 1 day as the fibers swell. The fibers arecollected by suction filtration and are washed with distilled water.Their wet weight is 3564.5 g and their consistency is determined to be1.43%, from which the Fiber Yield is calculated to be 51.0 g of dryfiber of the invention. The form is absorbent, though not necessarily100% of the cations inherently present are sodium: there may be hydrogencations present. The Conversion is about 94%. The WAARV of the fiber isdetermined as 94.8 g/g.

EXAMPLE IX Preparation of fiber of the invention

The procedure of Example VII is repeated except that the curing time is11.0 minutes. The procedure yields fibers having a wet weight of 234.0 gand consistency of 1.755%, from which the Fiber Yield is calculated tobe 4.11 g. The conversion is about 95%. The WAARV of the fiber isdetermined to be 86.8 g/g.

EXAMPLE X Preparation of fiber of the invention

The procedure of Example VII is repeated except that thepoly(arcylate-co-itaconate) is the product of Example I, the cellulosicfiber is chemithermomechanical pulp and the curing time at 130° is 10.0minutes. The procedure yields fibers having a wet weight of 170.92 g andconsistency of 2.58%, from which the Fiber Yield is calculated to be4.40 g. The Conversion is about 102% conversion. (percentage in excessof 100% is a consequence of expressing starting-materialpoly(acrylate-co-itaconate) on an acid basis whereas the productcontains additional sodium ions). The WAARV is determined to be 79.6g/g.

EXAMPLE XI Preparation of fiber of the invention

The procedure of Example VII is repeated except thatpoly(acrylate-co-itaconate) from example I (1.00 g) dissolved in 10 mlof water, polyethylene glycol with a nominal molecular weight of 3350(0.150 g), and chemithermomechanical pulp (1.00 g on a bone-dry basis)are used. The pH of the aqueous medium is 2.00 and the curing time is14.0 minutes at a curing temperature of 130° C. The acid-form fibers arerepulped in a Waring Blendor for 1 minute on low speed. The procedureyields fibers having a wet weight of 130.24 g and consistency of 1.59%,from which the Fiber Yield is calculated to be 2.07 g. The Conversion isabout 96%. The WAARV is determined to be 51.7 g/g.

EXAMPLE XII Preparation of fiber of the invention

The procedure of Example VII is repeated with the following exceptions:poly(acrylate-co-itaconate) is from Example V; the starting-materialcellulosic fiber is chemithermomechanical pulp; the pH of the aqueousmedium is 2.00; and the curing time is 15.0 minutes at a curingtemperature of 130° C. After curing, the acid-form fibers are repulpedin a Waring Blendor for 1 minute on low speed. The procedure yieldsfibers having a wet weight of 181.56 g and consistency of 2.09%, fromwhich the Fiber Yield is calculated to be 3.79 g. The Conversion isabout 88%. The WAARV is determined to be 46.1 g/g.

EXAMPLE XIII Preparation of fiber of the invention

The procedure in Example VII is repeated with the following exceptions:poly(acrylate-co-itaconate) is from Example V; the cellulosic fiber usedas starting-material is chemithermomechanical pulp, and the curing timeat 130° C. is 10.0 minutes. After curing, the acid-form fibers arerepulped in a Waring Blendor for 1 minute on low speed. The procedureyields fibers having a wet weight of 205.93 g and consistency of 1.83%,from which the Fiber Yield is calculated to be 3.77 g. The Conversion isabout 87%. The WAARV is determined to be 77.2 g/g.

EXAMPLE XIV Preparation of fiber of the invention

The procedure in Example VII is repeated with the following exceptions:poly(acrylate-co-itaconate) is from Example V and the curing time at130° C. is 10.0 minutes. After curing, the acid-form fibers are repulpedin a Waring Blendor for 1 minute on low speed. The procedure yieldsfibers having a wet weight of 238.86 g and consistency of 1.72%, fromwhich the Fiber Yield is calculated to be 4.11 g. The Conversion isabout 96%. The WAARV is determined to be 97.8 g/g.

EXAMPLE XV Preparation of fiber of the invention

The procedure in Example VII is repeated with the following exceptions:poly(acrylate-co-itaconate) is from Example VI (16.67 g of the 12%solids solution are used); the cellulosic fiber used asstarting-material is chemithermomechanical pulp; the pH of the aqueousmedium is 2.00; and the curing time at 130° C. is 14.0 minutes. Aftercuring, the acid-form fibers are repulped in a Waring Blendor for 1minute on low speed. The procedure yields fibers having a wet weight of230.0 and consistency of 1.61%, from which the Fiber Yield is calculatedto be 3.71 g. The Conversion is about 86%. The WAARV is determined to be97.8 g/g.

EXAMPLE XVI Preparation of fiber of the invention

The procedure in Example VII is repeated with the following exceptions:poly(acrylate-co-itaconate) is from Example VI (16.67 g of the 12%solids solution are used); the cellulosic fiber used asstarting-material is chemithermomechanical pulp; and the curing time at130° C. is 10.0 minutes. The procedure yields fibers having a wet weightof 230.52 g and consistency of 1.76%, from which the Fiber Yield iscalculated to be 4.06 g. The Conversion is about 94%. The WAARV isdetermined to be 82.6 g/g.

EXAMPLE XVII Preparation of fiber of the invention

The procedure of Example VII is repeated with the following exceptions:poly(acrylate-co-itaconate) is from Example II and the curing time at130° C. is 11.0 min. The procedure yields fibers having a wet weight of266.02 g and consistency of 1.455%, from which the Fiber Yield iscalculated to be 3.87 g. The Conversion is about 89%. The WAARV isdetermined to be 99.4 g/g.

EXAMPLE XVIII Preparation of fiber of the invention

The procedure of Example VII is repeated with the following exceptions:poly(acrylate-co-itaconate) is from Example II and the curing time at130° C. is 12.0 min. The procedure yields fibers having a wet weight of120.73 g and consistency of 3.81%, from which the Fiber Yield iscalculated to be 4.60 g. The Conversion is about 106%. The WAARV isdetermined to be 62.8 g/g.

EXAMPLE XIX Preparation of fiber of the invention

The procedure of Example VII is repeated with the following exceptions:poly(acrylate-co-itaconate) is from Example II and the curing time at130° C. is 13.0 min. The procedure yields fibers having a wet weight of101.06 g and consistency of 4.53%, from which the Fiber Yield iscalculated to be 4.57 g. The Conversion is about 106%. The WAARV isdetermined to be 49.2 g/g.

EXAMPLE XX Preparation of fiber of the invention

The procedure of Example VII is repeated with the following exceptions:poly(acrylate-co-itaconate) is from Example II and the curing time at130° C. is 14.0 min. The procedure yields fibers having a wet weight of110.12 g and consistency of 4.19%, from which the Fiber Yield iscalculated to be 4.61 g. The Conversion is about 106%. The WAARV isdetermined to be 43.2 g/g.

EXAMPLE XXI Preparation of fiber of the invention

The procedure of Example VII is repeated with the exception that the pHof the aqueous medium is 2.5. The procedure yields fibers having a wetweight of 314.51 g and consistency of 1.164%, from which the Fiber Yieldis calculated to be 3.66 g. The Conversion is about 85%. The WAARV isdetermined to be 125.8 g/g.

EXAMPLE XXII Preparation of fiber of the invention

The procedure of Example VII is repeated with the exception that the pHof the aqueous medium is 3.5. The procedure yields fibers having a wetweight of 110.99 g and consistency of 3.955%, from which the Fiber Yieldis calculated to be 4.39 g. The Conversion is about 100%. The WAARV isdetermined to be 17.8 g/g.

EXAMPLE XXIII Preparation of fiber of the invention

The procedure of Example VII is repeated except that the pH of theaqueous medium is 4.0. The procedure yields fibers having a wet weightof 185.36 g and consistency of 2.25%, from which the Fiber Yield iscalculated to be 4.17 g. The Conversion is about 96%. The WAARV isdetermined to be 43.1 g/g.

EXAMPLE XXIV Preparation of fiber of the invention

The procedure of Example VII is repeated with the exception thatpolyethylene glycol with a nominal molecular weight of 200 (0.060 g) isused as the polyol. The procedure yields fibers having a wet weight of128.57 g and consistency of 2.88%, from which the Fiber Yield iscalculated to be 3.71 g. The Conversion is about 91%. The WAARV isdetermined to be 21.9 g/g.

EXAMPLE XXV Preparation of fiber of the invention

The procedure of Example VII is repeated except that polyethylene glycolwith a nominal molecular weight of 1000 (0.100 g) is used as the polyol.The procedure yields fibers having a wet weight of 228.00 g andconsistency of 1.56%, from which the Fiber Yield is calculated to be3.56 g. The Conversion is about 87%. The WAARV is determined to be 50.8g/g.

EXAMPLE XXVI Preparation of fiber of the invention

The procedure of Example VII is repeated except that polyethylene glycolwith a nominal molecular weight of 1500 (0.150 g) is used as the polyol.The procedure yields fibers having a wet weight of 211.74 g andconsistency of 1.85%, from which the Fiber Yield is calculated to be3.91 g. The Conversion is about 94%. The WAARV is determined to be 83.7g/g.

EXAMPLE XXVII Preparation of fiber of the invention

The procedure of Example VII is repeated except that polyethylene glycolwith a nominal molecular weight of 6800 (0.500 g) is used as the polyol.The procedure yields fibers having a wet weight of 138.48 g andconsistency of 2.87%, from which the Fiber Yield is calculated to be3.98 g. The Conversion is about 88%. The WAARV is determined to be 76.8g/g.

EXAMPLE XXVIII Preparation of fiber of the invention

The procedure of Example VII is repeated with the following exceptions:poly(acrylate-co-itaconate) copolymer is from Example II (1.00 gdissolved in 20 ml of water); the polyol is polyethylene glycol with anominal molecular weight of 3350 (0.100 g); the starting-materialcellulosic fiber is chemithermomechanical pulp (2.00 g on a bone-drybasis); the pH of the aqueous medium is 2.00 and the curing time at 130°C. is 14.0 minutes. After curing, the fibers are repulped in a WaringBlendor for 1 minute on low speed. The procedure yields fibers having awet weight of 115.30 g and consistency of 2.54%, from which the FiberYield is calculated to be 2.93 g. The Conversion is about 95%. The WAARVis determined to be 24.1 g/g.

EXAMPLE XXIX Preparation of fiber of the invention

The procedure of Example VII is repeated with the following exceptions:poly(acrylate-co-itaconate) is from Example II (1.80 g dissolved in 30ml of water); the polyol is polyethylene glycol with a nominal molecularweight of 3350 (0.300 g); the starting-material cellulosic fiber ischemithermomechanical pulp (3.00 g on a bone-dry basis); the pH of theaqueous medium is 2.00 and the curing time at 130° C. is 14.0 minutes.After curing, the chemically modified fibers are repulped in a WaringBlendor for 1 minute on low speed. The procedure yields fibers having awet weight of 186.77 g and consistency of 2.57%, from which the FiberYield is calculated to be 4.80 g. The Conversion is about 95%. The WAARVis determined to be 35.9 g/g.

EXAMPLE XXX Preparation of wet-laid paper comprising fiber of theinvention (Example VII) in admixture with conventional fiber

A slurry of northern softwood Kraft pulp (NSK) is prepared by repulpingNSK dry-lap (1.75 g bone-dry basis) in 400 ml of distilled water in aWaring Blendor on low speed for 1.0 minute. The slurry is placed in a 1liter beaker and to it is added fiber of the invention (sodium form,made according to Example VII but never dried, 0.75 g bone-dry basis, in100 ml of distilled water). The pH of the slurry is adjusted to 8.5 with0.1 Molar sodium hydroxide and the slurry is stirred for 1 hour. Adeckle box is fitted with a forming wire (Albany International-AppeltonWire Division, Appelton, Wis.; Handsheet style/mesh 78-S) and is filledwith distilled water which is also adjusted to pH 8.5 with 1 Molarsodium hydroxide. The slurry is added and the water is drained bysuction. The wet paper sheet (handsheet) thus formed is transferred to adrying fabric (albany International-Appelton Wire Division, Appelton,Wis.; Handsheet style/mesh 36-C) by passage over a vacuum slit on lowsetting. The drying fabric is passed over the vacuum slit two additionaltimes on high setting and then another fabric is placed on top of thewet handsheet. The sandwhich is passed through a drum dryer at 230° F.until the sheet is dry. This gives a 2.50 g handsheet (basis weight=16.5lbs/3,000 square feet) containing 30% by weight of fiber of theinvention. This handsheet is quick-wicking and has a WAARV of 22.6 g/g.The handsheet can be wetted and re-dried: on rewet, it is found to havepreserved good absorbency and wicking characteristics.

EXAMPLE XXXI Preparation of wet-laid paper comprising fiber of theinvention (Example VIII) in admixture with conventional fiber.

The procedure of Example XXX is repeated except that the fiber of theinvention of Example VIII is used. The WAARV of the handsheet isdetermined to be 10.7 g/g.

EXAMPLE XXXII Preparation of wet-laid paper comprising fiber of theinvention (Example IX) in admixture with conventional fiber

The procedure of Example XXX is repeated except that the fiber of theinvention of Example IX is used. The WAARV of the handsheet isdetermined to be 24.7 g/g.

EXAMPLE XXXIII Preparation of wet-laid paper comprising fiber of theinvention (Example X) in admixture with conventional fiber

The procedure of Example XXX is repeated except that the fiber of theinvention of Example X is used. The WAARV of the handsheet is determinedto be 23.7 g/g.

EXAMPLE XXXIV Preparation of wet-laid paper comprising fiber of theinvention (Example XI) in admixture with conventional fiber

The procedure of Example XXX is repeated except that the fiber of theinvention of Example XI is used. The WAARV of the handsheet isdetermined to be 20.7 g/g.

EXAMPLE XXXV Preparation of wet-laid paper comprising fiber of theinvention (Example XII) in admixture with conventional fiber

The procedure of Example XXX is repeated except that the fiber of theinvention of Example XII is used. The WAARV of the handsheet isdetermined to be 19.8 g/g.

EXAMPLE XXXVI Preparation of wet-laid paper comprising fiber of theinvention (Example XIII) in admixture with conventional fiber

The procedure of Example XXX is repeated except that the fiber of theinvention of Example XIII is used. The WAARV of the handsheet isdetermined to be 18.7 g/g.

EXAMPLE XXXVII Preparation of wet-laid paper comprising fiber of theinvention (Example XIV) in admixture with conventional fiber

The procedure of Example XXX is repeated except that the fiber of theinvention of Example XIV is used. The WAARV of the handsheet isdetermined to be 15.5 g/g.

EXAMPLE XXXVIII Preparation of wet-laid paper comprising fiber of theinvention (Example XV) in admixture with conventional fiber

The procedure of Example XXX is repeated except that the fiber of theinvention of Example XV is used. The WAARV of the handsheet isdetermined to be 29.2 g/g.

EXAMPLE XXXIX Preparation of wet-laid paper comprising fiber of theinvention (Example XVI) in admixture with conventional fiber

The procedure of Example XXX is repeated except that the fiber of theinvention of Example XVI is used. The WAARV of the handsheet isdetermined to be 19.8 g/g.

EXAMPLE XL Preparation of wet-laid paper comprising fiber of theinvention (Example XVII) in admixture with conventional fiber

The procedure of Example XXX is repeated except that the fiber of theinvention of Example XVIII is used. The WAARV of the handsheet isdetermined to be 18.6 g/g.

EXAMPLE XLI Preparation of wet-laid paper comprising fiber of theinvention (Example XVIII) in admixture with conventional fiber

The procedure of Example XXX is repeated except that the fiber of theinvention of Example XVIII is used. The WAARV of the handsheet isdetermined to be 9.6 g/g.

EXAMPLE XLII Preparation of wet-laid paper comprising fiber of theinvention (Example XIX) in admixture with conventional fiber

The procedure of Example XXX is repeated except that the fiber of theinvention of Example XIX is used. The WAARV of the handsheet isdetermined to be 7.0 g/g.

EXAMPLE XLIII Preparation of wet-laid paper comprising fiber of theinvention (Example XX) in admixture with conventional fiber

The procedure of Example XXX is repeated except that the fiber of theinvention of Example XX is used. The WAARV of the handsheet isdetermined to be 5.7 g/g.

EXAMPLE XLIV-LII Preparation of wet-laid paper comprising fiber of theinvention

The procedures of Examples XXX-XLIII are repeated except that 2.00 g ofnorthern softwood Kraft dry-lap (bone-dry basis) and 0.50 g of the fiberof the invention (bone-dry basis) are used. This give 2.50 g handsheets(basis weight=16.5 lbs./3,000 square feet) containing 20% by weight ofthe fiber of the invention. The results are as follows.

    ______________________________________                                        Example No.  Handsheet WAARV (g/g)                                            ______________________________________                                        XLIV         15.6                                                             XLV           8.5                                                             XLVI         13.2                                                             XLVII        14.0                                                             XLVIII       10.0                                                             XLIX         12.8                                                             L            11.6                                                             LI           11.8                                                             LII          19.0                                                             LIII         13.1                                                             LIV          13.4                                                             LV            6.4                                                             LVI           6.8                                                             LVII          5.6                                                             ______________________________________                                    

TEST METHODS Weight average molecular weight ofpoly(acrylate-co-itaconate)

Weight average molecular weights, M_(W), of copolymer samples aredetermined by low angle laser light scattering using a KMX-6 ChromatixPolymer Analyzer (flow injection method). The change in refractive indexwith concentration, d_(n) /d_(c), is measured on a KMX-16 LaserDifferential Refractometer at 25° C. after the copolymer solutions aredialyzed against 0.2 Molar sodium chloride in water. The intercept of alinear regression analysis of a plot of K_(C) /R_(theta) versus c isgiven by 1/M_(W), where K=a(n)² (d_(n) /d_(c))² (the constant a beingcharacteristic of the particular instrument), c=concentration ofcopolymer, and R_(theta) is the Rayleigh scattering for a givencopolymer solution. Typically, the concentrations of copolymer used formolecular weight determinations are 1.0, 1.5, 2.0, 2.5, and 3.0 mg/ml.

pH of aqueous medium; pH in general

In general, pH herein is determined using a conventional digital pHmeter which has an accuracy of ±0.01 pH units (Markson model 88). Themeter is equipped with a flat surface electrode, which has a peripheralporous polyethylene junction (Markson 12008B). The electrode isparticularly suited for measuring the pH of slurries, viscous solutionsand wet surfaces. As an alternative, a conventional polymer-gel-filledpH electrode may be used. pH measurements are made at ambienttemperature, in the range 20° C.-25° C. The electrodes are calibrated inthe conventional manner, using pH 7.00 and pH 4.00 buffers.

It is specifically noted that the above-identified equipment andprocedure is used for measuring pH of the aqueous medium discussedhereinabove in the specification.

Consistency

Consistency, such as of wet fiber of the invention, is defined aspercentage by weight of a specified fiber, in a slurry, fiber dispersionor wet fiber mass. Measurement is carried out by placing a sample of wetmaterial sufficient to give at least about 0.1 gram of bone dry fiber ona Mettler PM460 balance which is equipped for moisture determination(infra-red dryer model LP16), weighing wet followed by continuousmonitoring of weight during drying (90° C. temperature setting) toconstant weight.

Fiber Yield

Fiber Yield is defined as the weight in grams, dry basis, of fiber ofthe invention, sodium form. It is conveniently measured by multiplyingthe weight of wet, swollen fibers of the invention by the consistency.

Conversion

Conversion is defined as the yield of fiber of the invention expressedin percentage terms. It is calculated by dividing the Fiber Yield by thesum of weights of starting-materials, more specifically the sum ofweights, bone dry basis, of poly(acrylate-co-itaconate) plus polyol pluscellulosic fiber starting-material.

In determining Conversion, the weight of poly(acrylate-co-itaconate) inthe above is expressed on an acid equivalent basis. That is to say,regardless of the form of the poly(acrylate-co-itaconate) used in thesynthesis of the fiber of the invention, and equally regardless of theform of the product fiber, the convention is adopted of everywherespecifying the poly(acrylate-co-itaconate) weight as though it were inthe acid form, i.e., all the charge-balancing cations are H. In thismanner, the relative proportion of poly(acrylate-co-itaconate) to thecellulose and polyol components is unambiguously determined.

As noted hereinabove, Conversion can be slightly in excess of 100%(typically up to about 106%) as a consequence of cation weight gain.Thus when the poly(acrylate-co-itaconate) starting-material is in theacid form and the fiber of the invention is secured in the sodium form,the heavier sodium cation as compared with hydrogen cations accounts forthe additional weight gain.

Water absorbency and retention value (WAARV) of fiber of the inventionand WAARV of wet-laid paper containing same

The following is a gravimetric water-absorbency and retention-measuringmethod applicable to characterizing the fibers, pulps or paper websaccording to the invention. For purposes of comparison, typicalpapermaking pulp such as Kraft pulp measures of the order of about 3-4grams of water per gram of fiber at pH 8.5 ("g/g") by this method andpaper webs made from such pulp have similar or slightly lower values.Equipment is as follows:

Sample holders: glass cylinders open at both ends, 1.8 cm. insidediameter, 4.2 cm height.

Tea-bag material: Tea-bag paper, grade 1234T, obtainable from C. H.Dexter Division of the Dexter Corp., Windsor Locks, Conn. This paper iscut into 4.7×9.5 cm rectangles. The purpose of the tea-bag material isto provide a substantially non-absorbent pulp-retaining material throughwhich water will pass during centrifugation, and which acts to preventthe possibility of obtaining artificially high pulp absorbencies, whichmight otherwise occur, e.g., if the pulp were allowed to block theconstriction in the centrifuge tube.

Balance: 0.0001 g sensitivity.

Centrifuge: clinical model, variable speed, with a swinging bucketrotor, four 29.4 mm. inside diameter×95 mm depth shields, and tachometeradapted to measure centrifuge speed.

Centrifuge tubes: designed with a constriction so that on centrifuging,the water will separate into the lower half of the tube, leaving thesample and "tea-bag" in the upper half.

Drying beakers: 10 ml capacity.

Vacuum oven: capable of approximately 250 mTorr vacuum, heating to atleast 110° C.; temperature thermostatted at 60° C.

Convection oven: thermostatted at 105° C.

Soaking beakers: 150 ml capacity.

For each absorbency determination, a number of replicated measurements(typically two will suffice provided that the results are in goodagreement) are made, each based on the following procedure:

Weigh a tea-bag paper. The weight is the Initial Teabag Weight (InitialTeabag Weight=ITB) and is typically of the order of 70 mg.

Place the fiber or paper (shredded in small pieces) which is to betested for absorbency into a 150 ml beaker. Add 100 ml distilled water.Adjust pH to 8.5 with aqueous sodium hydroxide. Equilibrate by allowingto stand for about 2 hours.

Fold a weighed tea-bag paper to make a cylindrically shaped holderhaving one end closed and the other end open. Place it inside a glasscylinder. Into the shaped tea-bag, place wet equilibrated material to betested, allowing excess water to drain through the tea-bag, until thetea-bag is substantially full with wet fiber. (Typically when the sampleto be tested in fiber of the invention, the sample size is sufficient tocontain about 100 mg bone dry fiber; when the sample to be tested is awet-laid paper containing the fiber of the invention, the sample size issufficient to contain about 300 mg in total of all fiber present; andwhen the sample to be tested is conventional fiber, e.g., northernsoftwood Kraft, the sample size is sufficient to contain about 500 mg offiber. Slip the tea-bag out of the glass cylinder or holder and,preserving the cylindrical shape of the tea-bag, place it and its samplecontents in a centrifuge tube. Centrifuge at approximately 125 "g"(gravities) force for 10 minutes, centifuge speed-up time not included.Place the centrifuged sample and tea-bag in an accurately preweighed drybeaker (dry beaker weight=DBW). Weigh the centrifuged sample, tea-bagand beaker (weight=W₁). Dry in the 105° C. convection oven for 3 hours.Further dry in the vacuum oven for 6 hours or more. Allow to cool in adesiccator. Weigh (weight=W₂). The water absorbency and retention value(WAARV) of the sample (g/g) is given by the following formula:

    WAARV=(WPW-DPW)/DPW

wherein WPW=wet pulp weight=W₁ -(ITB+DBW) and DPW=dry pulp weight=W₂-(ITB+DBW). In principle, it is possible to measure WAARV absorbency atpH values other than 8.5 of the above-specified method. However, unlessthere is a specific mention of another pH, any WAARV absorbency valuequoted throughout the instant specification and claims, expressed simplyin g/g, is strictly to be construed as a measurement at a pH of 8.5.

What is claimed is:
 1. A chemically modified fiber having a waterabsorbency and retention value in the range from about 15 g/g to about130 g/g comprising, chemically bonded together:(a) a cellulosic fiberselected from the group consisting of chemithermomechanical pulp fiber,bleached hardwood Kraft pulp fiber, bleached softwood Kraft pulp fiber,unbleached hardwood Kraft pulp fiber, unbleached softwood Kraft pulpfiber, bleached softwood sulfite pulp fiber, unbleached softwood sulfitepulp fiber, cotton linters, mercerized dissolving pulp fiber,unmercerized dissolving pulp fiber, and mixtures thereof; (b) apoly(acrylate-co-itaconate) having a weight average molecular weight inthe range from about 60,000 to about 1,000,000, an acrylate content offrom about 50 mole % to about 99 mole % and an itaconate content of fromabout 1 mole % to about 50 mole %, and (c) a polyol;wherein theproportion by weight of said poly(acrylate-co-itaconate) to polyol isfrom about 250:1 to about 3:1 and the weight of saidpoly(acrylate-co-itaconate) plus said polyol per unit weight of saidcellulosic fiber, (a), is in the range from about 0.3 to about 2, thepoly(acrylate-co-itaconate) weight being expressed on an acid equivalentbasis.
 2. A chemically modified fiber according to claim 1 wherein saidcellulosic fiber, (a), is selected from the group consisting ofchemithermomechanical pulp fiber, bleached hardwood Kraft pulp fiber,bleached softwood Kraft pulp fiber, unbleached hardwood Kraft pulpfiber, unbleached softwood Kraft pulp fiber, bleached softwood sulfitepulp fiber, unbleached softwood sulfite pulp fiber, and mixturesthereof; said poly(acrylate-co-itaconate) has weight average molecularweight in the range from about 400,000 to about 1,000,000; said acrylatecontent is from about 70 mole % to about 98 mole %; said itaconatecontent is from about 2 mole % to about 30 mole %; and said polyol isselected from the group consisting of polethylene glycol, polyvinylalcohol, ethylene glycol, propylene glycol, glycerin andpentaerythritol.
 3. A chemically modified fiber according to claim 2wherein said polyol has formula HO(CH₂ CH₂ O)_(n) H wherein n is fromabout 4 to about 154 and said proportion by weight ofpoly(acrylate-co-itaconate) to polyol is from about 30:1 to about 4:1.4. A chemically modified fiber according to claim 3 wherein saidpoly(acrylate-co-itaconate) has acrylate content of from about 90 mole %to about 95 mole % and itaconate content of from about 5 mole % to about10 mole %; said weight average molecular weight is in the range fromabout 600,000 to about 900,000; n in said formula is from about 34 toabout 100; and said weight of poly(-acrylate-co-itaconate) plus polyolper unit weight of said cellulosic fiber, (a), is in the range fromabout 0.6 to about 1.5.
 5. A chemically modified fiber according toclaim 4 wherein said water absorbency and retention value is in therange from about 30 g/g to about 100 g/g.
 6. A chemically modified fiberaccording to claim 5 wherein n is said formula is from about 70 to about80; said proportion by weight of poly(acrylate-co-itaconate) to polyolis from about 10:1 to about 5:1; and said weight ofpoly(acrylate-co-itaconate) plus polyol per unit weight of cellulosicfiber, (a), is in the range from about 0.8 to about 1.2.
 7. A chemicallymodified fiber according to claim 6 wherein said water absorbency andretention value is in the range from about 50 g/g to about 90 g/g.
 8. Achemically modified fiber according to claim 7 wherein the cations,which are inherently present in a charge-balancing amount, are selectedfrom the group consisting of sodium, potassium, lithium, hydrogen andmixtures thereof.
 9. A chemically modified fiber according to claim 8wherein said cations are selected from the group consisting of sodium,hydrogen and mixtures thereof.
 10. A cellulosic papermaking pulpconsisting essentially of the chemically modified fiber of claim
 1. 11.A cellulosic papermaking pulp consisting essentially of the chemicallymodified fiber of claim
 9. 12. A cellulosic papermaking pulp accordingto claim 11 wherein the content of cations which are hydrogen is such asto produce a pH of less than 5 when dispersed in water.
 13. A cellulosicpapermaking pulp according to claim 11 wherein the content of cationswhich are hydrogen is such as to produce a pH of about 6 to about 9 whendispersed in water.
 14. A cellulosic papermaking pulp consistingessentially of:from about 5% to about 60% of the chemically modifiedfibers of claim 11 and from about 40% to about 95% of conventionalcellulosic fiber.